A typical example of the fall board assembly provided in a piano like keyboard instrument is illustrated in FIG. 1 of the drawings and largely comprises a fall board 1 provided between two side arms one of which is indicated by phantom line 2, two sets of pivotal units each consisting of a fall board pivot 3 attached to each of the side surfaces of the fall board 1 and a fall board pivot plate 4 attached to each of the side arms and balancing weight 5 attached to the fall board 1 in spacing relation from the fall board pivot 3. The fall board 1 thus arranged is driven for rotation about the center axes of the fall board pivots and positioned into a closed state or an open state. When the fall board 1 is positioned from the closed state into the open state as shown in FIG. 1, the keyboard consisting of a plurality of keys 6 are exposed to a player, thereby being ready for performance.
The fall board assembly illustrated in FIG. 1 has the balancing weight 5 urging the fall board 1 to the open state, so that the fall board 1 tends to keep the open state against a moment due to the weight thereof even if the fall board 1 slightly loses the balance. Moreover, the fall board 1 is prevented from rapid falling and slowly rotates toward the closed state by virtue of the balancing weight 5.
FIG. 2 shows another example of the fall board assembly which largely comprises a fall board 11 formed with two generally crescent-shaped slots one of which is designated by reference numeral 12, two sets of pivotal units 13 allowing the fall board to be rotatable about the center axes thereof, and two leaf springs 14 fixed at the rear end portions thereof to a pair of key blocks 15 and inserted into the generally crescent-shaped slots 12, respectively. The leaf springs 14 are in contact with the respective lower surfaces 16 partially defining the generally crescent-shaped slots 12, respectively, and urge the fall board 1 to be driven for rotation toward the open state thereof, so that the fall board slowly rotates toward the closed state when pulling down by virtue of the resilient force of the leaf springs 14.
However, a problem is encountered in each prior-art fall board assembly in insufficient absorption of force at the final stage of rotation of the fall board. This is because of the fact that the counter moment applied to the fall board due to the weight 5, or the leaf spring 16 is substantially constant over the rotation of the fall board. Namely, if a relatively small counter moment is applied to the fall board for easy pulling motion at the initial stage, the fall board should be strongly supported by a player at the final stage of the rotation for preventing the fall board from crashing against the key slip. The fall board assembly equipped with the leaf springs 14 has an additional problem in complex assembly, because the pivotal units and the leaf springs should be simultaneously assembled.